Metal chelates are useful for improving the contrast of X-ray, ultrasound, radionuclide and magnetic resonance (MR) images. However, the metal complexes must be stable and inert so that toxicity resulting from dissociation in the body can be minimized. Further, highly stable chelates of paramagnetic ions, like gadolinium(III), are used to reduce the longitudinal and transverse relaxation times T1 and T2 of water in MRI. Samarium (III), particularly 153Sm can emit both gamma radiation for imaging and beta radiation for localized radiation.
A variety of chelates, including derivatives of DTPA (diethylenetriaminepentaacetic acid) or DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) were used to chelate Gd3+ ions. However, such chelates are not charge balanced and such may lower the stability of the resulting metal complex.